Pathogenic features of heterotrophic plate count bacteria
from drinking-water boreholes
Suranie Horn, Rialet Pieters and Carlos Bezuidenhout
ABSTRACT
Evidence suggests that heterotrophic plate count (HPC) bacteria may be hazardous to humans with
weakened health. We investigated the pathogenic potential of HPC bacteria from untreated borehole
water, consumed by humans, for: their haemolytic properties, the production of extracellular
enzymes such as DNase, proteinase, lipase, lecithinase, hyaluronidase and chondroitinase, the effect
simulated gastric fluid has on their survival, as well as the bacteria’s antibiotic-susceptible profile.
HuTu-80 cells acted as model for the human intestine and were exposed to the HPC isolates to
determine their effects on the viability of the cells. Several HPC isolates were α- or β-haemolytic,
produced two or more extracellular enzymes, survived the SGF treatment, and showed resistance
against selected antibiotics. The isolates were also harmful to the human intestinal cells to varying
degrees. A novel pathogen score was calculated for each isolate. Bacillus cereus had the highest
pathogen index: the pathogenicity of the other bacteria declined as follows: Aeromonas
taiwanensis > Aeromonas hydrophila > Bacillus thuringiensis > Alcaligenes faecalis > Pseudomonas
sp. > Bacillus pumilus > Brevibacillus sp. > Bacillus subtilis > Bacillus sp. These results demonstrated
that the prevailing standards for HPCs in drinking water may expose humans with compromised
immune systems to undue risk.
Suranie Horn (corresponding author)
Rialet Pieters
Carlos Bezuidenhout
Unit for Environmental Sciences and Management,
Potchefstroom Campus, North-West University,
Private Bag X6001,
Potchefstroom 2520,
South Africa
E-mail: suraans@gmail.com
Key words | cytotoxicity, extracellular enzymes, HPC bacteria, human duodenal cells, pathogenicity,
simulated gastric fluid
INTRODUCTION
There is evidence that heterotrophic bacteria are dangerous
to human health (Edberg & Allen ; Pavlov et al. )
and may contribute to what is generally referred to as
acute gastrointestinal illness (AGI), resulting in fever,
nausea and diarrhoea and/vomiting (Macler & Merkle
). Most of the AGI are acute, self-resolving and do not
have major consequences to healthy individuals. This is
however not the case for immuno-compromised individuals.
Heterotrophic bacteria use organic nutrients as their
energy source and are present in water, air, soil and food
(Edberg & Allen ). Heterotrophic plate count (HPC)
bacteria are a subset of heterotrophic bacteria and can be
isolated in the laboratory by using culture-based methods
under a predetermined set of conditions (WHO ).
There is much controversy over the usefulness of HPC
bacteria as indicators of microbial water quality (Stelma
et al. ; Donskey ). Previous studies investigated the
potentially pathogenic features of HPC isolates (Pavlov
et al. ). The authors of this paper subscribe to the view
of Casadevall & Pirofski () that pathogenicity refers to
the ability to cause disease mediated by specific virulence fac-
tors. A number of studies reported HPC bacteria to have
virulent characteristics associated with potential pathogen-
icity such as haemolysis, secretion of extracellular enzymes
(Pavlov et al. ), which cause them to be cytotoxic to
cells (Lye & Dufour ), to adhere to cells (Pavlov et al.
), and to survive passing through the gastric fluids of
the stomach ( Janda & Bottone ; Yuk & Marshall ).
890 © IWA Publishing 2016 Journal of Water and Health | 14.6 | 2016
doi: 10.2166/wh.2016.009
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